Aqueous processing liquid

ABSTRACT

An aqueous processing liquid used for cutting a fragile material using a wire saw contains a water-soluble polymer, in which the water-soluble polymer has a mass average molecular weight in a range of 3×10 3  to 6×10 5 , and the aqueous processing liquid exhibits a viscosity at 25 degrees C. in a range of 1.5 mPa·s to 15 Pa·s.

TECHNICAL FIELD

The present invention relates to an aqueous processing liquid, morespecifically, an aqueous processing liquid used for cutting a fragilematerial with a fixed abrasive grain wire saw.

BACKGROUND ART

In manufacturing semiconductor products, a silicon ingot (that isfragile in nature) needs to be cut. In such a case, a wire sawprocessing is generally employed in terms of cutting accuracy andproductivity. Herein, a cutting method of the silicon ingot includes: aloose abrasive grain method of cutting the silicon ingot with aprocessing liquid in which grains are dispersed; and a fixed abrasivegrain method of cutting the silicon ingot with a wire having grainsfixed on its surface in advance.

A processing liquid used for the loose abrasive grain method isexemplified by an aqueous processing liquid containing afriction-coefficient reducer, an anticorrosion adjuvant and the like. Anunsaturated fatty acid is used as the friction-coefficient reducercontained in the processing liquid and benzotriazole is used as theanticorrosion adjuvant (see Patent Literature 1). According to the looseabrasive grain method, a large margin is required when a thick wire isused, so that a large amount of cut powder is generated to deteriorate ayield after cutting the ingot. In addition, since the wire becomes wornas being used, there is a limit to reduce a diameter of the wire itself.Accordingly, the loose abrasive grain method is not productive in themanufacturing a silicon wafer for a solar battery and the like, which isexpected to be significantly increasingly produced in the future.

On the other hand, a water soluble processing liquid containing glycolshas been known as the processing liquid used in the fixed abrasive grainmethod (see Patent Literatures 2 and 3). In the fixed abrasive grainmethod, since grains are fixed to the wire in advance, the diameter ofthe wire is reducible and the cut powder is less producible, therebyresulting in an excellent productivity.

CITATION LIST Patent Literature(s)

Patent Literature 1: JP-A-8-57848

Patent Literature 2: JP-A-2003-82334

Patent Literature 3: JP-A-2011-21096

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

In the processing of obtaining the wafer by cutting the silicon ingotwith the wire saw, a low production cost is severely demanded and thediameter of the wire is desired to be as small as possible in order toincrease a yield of the wafer. However, when the diameter of the wire issmall, since the ingot needs to be cut by a low tension, the wire may beeasily deflected to decrease the cutting accuracy. The above-describedproblems cannot be fully solved by the fixed abrasive grain method usingthe water soluble processing liquid as disclosed in Patent Literatures 2and 3.

An object of the invention is to provide an aqueous processing liquidcapable of inhibiting deflection of a small-diameter wire to provide afavorable cutting accuracy, even when a fragile material is cut with thewire.

Means for Solving the Problems

In order to solve the above problems, an aspect of the inventionprovides the following water soluble processing liquid.

(1) In an aspect of the invention, an aqueous processing liquid used forcutting a fragile material using a wire saw contains a water-solublepolymer, in which the water-soluble polymer has a mass average molecularweight in a range of 3×10³ to 6×10⁵, and the aqueous processing liquidexhibits a viscosity at 25 degrees C. in a range of 1.5 mPa·s to 15Pa·s.

(2) In the aqueous processing liquid in the above aspect of theinvention, the wire saw is a fixed abrasive grain wire saw.

(3) In the aqueous processing liquid in the above aspect of theinvention, the fixed abrasive grain wire saw has a wire diameter of 0.2mm or less.

(4) In the aqueous processing liquid in the above aspect of theinvention, a content of the water-soluble polymer is in a range of 0.5mass % to 40 mass % of a total amount of the processing liquid.

(5) The aqueous processing liquid in the above aspect of the inventionfurther contains at least one of a rust inhibitor, friction modifier,antifoaming agent, metal deactivator, bactericide (preservative) and pHadjuster.

(6) In the aqueous processing liquid in the above aspect of theinvention, the fragile material is a silicon ingot.

According to the aqueous processing liquid of the invention, even when afragile material is cut with a small-diameter wire, deflection of thewire can be inhibited to provide a high cutting accuracy. The aqueousprocessing liquid of the invention is particularly preferably usable fora fixed abrasive grain wire saw.

DESCRIPTION OF EMBODIMENT(S)

An aqueous processing liquid in an exemplary embodiment (hereinafter,simply referred to as “the present processing liquid”) is used inprocessing a fragile material with a wire saw and contains apredetermined water-soluble polymer.

Accordingly, a main component of the present processing liquid is water.Any water is usable without limitation, but purified water is preferablyused and deionized water is particularly preferable. A content of wateris preferably in a range of 50 mass % to 99 mass %, more preferably in arange of 60 mass % to 95 mass % of a total amount of the presentprocessing liquid. At the content of 50 mass % or more, sinceinflammability of the present processing liquid is decreased, safetythereof is increased. Further, the content of 50 mass % or more isfavorable in terms of an energy-saving performance and an environmentalaspect. The upper limit of water is preferably determined at 99 mass %or less in relation to contents of other components.

Note that the present processing liquid may be prepared by blending thecomponents (e.g., the water-soluble polymer) each by a requiredconcentration at an initial step, or alternatively, a concentratedliquid (stock solution) may be prepared and then diluted in use. Theconcentrated liquid preferably has a volume magnification ofapproximately twice to 160 times in terms of handleability.

A viscosity at 25 degrees C. of the present processing liquid is in arange of 1.5 mPa·s to 15 Pa·s, preferably in a range of 2 mPa·s to 10mPa·s, more preferably in a range of 3 mPa·s to 8 mPa·s. At less than1.5 mPa·s of the viscosity, adhesion to the wire is decreased to cause apoor lubrication, thereby decreasing the cutting accuracy. In contrast,at an excessively high viscosity, since an oil film on a surface of thewire is thickened during the cutting of the wire, bite of grains to aworkpiece is deteriorated. Consequently, the wire is heavily deflectedto decrease the cutting accuracy.

A mass average molecular weight of the water soluble polymer used in thepresent processing liquid is preferably in a range of 3×10³ to 6×10⁵,more preferably in a range of 5×10³ to 5×10⁵, further preferably in arange of 7×10³ to 3×10⁵, most preferably in a range of 7×10³ to 1×10⁵.

Since a viscosity-thickening effect is improved at 3×10³ or more of themass average molecular weight, the present processing liquid easilyadheres to the wire to enter an interval between the wire and thematerial. Accordingly, since peeling of the grains from the wire can beeffectively inhibited, the cutting speed can be increased. However, atmore than 6×10⁵ of the mass average molecular weight, a molecular chainis likely to be cut by shear, which may cause a large decrease in theviscosity. Accordingly, with respect to the cutting accuracy by thewire, a processing liquid containing a water-soluble polymer having sucha high molecular weight becomes inferior to a processing liquidcontaining a water-soluble polymer having an adequate molecular weight,even if both of the processing liquid has the same viscosity.

The water-soluble polymer preferably has an oxygen-containing group interms of processability. Examples of the oxygen-containing group are acarboxyl group, hydroxyl group, oxyethylene group, and oxypropylenegroup. Herein, the carboxyl group and the hydroxyl group includecarboxyl group and hydroxyl group in a form of an anion obtained bydeprotonation or neutralization.

Examples of the water-soluble polymer are poly(meth)acrylates, metalsalts (e.g., Na, K) of poly(meth)acrylates, (meth)acrylate-maleatecopolymer or metal salts (e.g., Na, K) thereof, (meth)acrylate-sulfonatecopolymer or metal salts (e.g., Na, K) thereof, polyoxyalkylene glycolsuch as polyoxyethylene glycol and polyoxypropylene glycol, polyvinylalcohol, a saponification product of an ethylene unsaturatedmonomer-vinyl acetate copolymer, and modified starch.

A content of the water-soluble polymer is preferably in a range of 0.5mass % to 40 mass % in the present processing liquid, more preferably ina range of 1 mass % to 20 mass %, further preferably in a range of 2mass % to 12 mass %. When the content of the water-soluble polymer fallswithin this range, the viscosity of the present processing liquid isalso controllable in a preferable range, so that advantages of theinvention is more easily exhibited.

Since the present processing liquid allows the fragile material (e.g.,silicon ingot) to be cut at a high speed while the grains are preventedfrom dropping off, the fragile material can be cut at a high accuracyeven using a fixed abrasive grain wire. Accordingly, the presentprocessing liquid is extremely effective in using the fixed abrasivegrain wire having a small diameter which requires cutting with a lowtension.

A diameter of the fixed abrasive grain wire saw is preferably 0.2 mm orless, more preferably 0.12 mm or less, further preferably 0.1 mm orless, most preferably 0.08 mm or less. At the smaller diameter of thewire saw, a yield at which a product is obtained from the fragilematerial (workpiece) can be increased. With the use of the presentprocessing liquid, since the bite of the grains to the workpiece isenhanced to increase a cutting efficiency, deflection of the wire saweven having a small diameter can be inhibited. However, the diameter ofthe wire saw is preferably 0.06 mm or more in terms of strength.

The present processing liquid may contain known additives such as a rustinhibitor, friction modifier, antifoaming agent, metal deactivator,bactericide (preservative) and pH adjuster as long as the effects of theinvention is not impaired.

Examples of the rust inhibitor are alkylbenzenesulfonate,dinonylnaphthalenesulfonate, alkenyl succinate and polyhydric alcoholester. A content of the rust inhibitor is preferably in a range ofapproximately 0.01 mass % to 5 mass % of a total amount of theprocessing liquid.

The friction modifier is used for inhibiting abrasion of the grains.Various surfactants are usable as the friction modifier. A non-ionicsurfactant (e.g., glycols) is a preferable example of the surfactants. Acontent of the surfactant is preferably in a range of approximately 0.01mass % to 5 mass % of the total amount of the processing liquid.

The antifoaming agent is used for preventing the processing liquid fromoverflowing from a processing liquid tank provided in a processing room.Examples of the antifoaming agent are silicone oil, fluorosilicone oil,and fluoroalkylether. A content of the antifoaming agent is preferablyin a range of approximately 0.001 mass % to 1 mass % of the total amountof the processing liquid.

Examples of the metal deactivator are imidazoline, pyrimidinederivative, thiadiazole, and benzotriazole. A content of the metaldeactivator is preferably in a range of approximately 0.01 mass % to 5mass % of the total amount of the processing liquid.

The bactericide (preservative) is used for preventing decomposition ofthe processing liquid. Examples of the bactericide (preservative) areparaoxy benzoic acid esters (parabens), benzoic acid, salicylic acid,sorbic acid, dehidroacetic acid, p-toluenesulfonic acids and saltsthereof, and phenoxyethanol. A content of the bactericide (preservative)is preferably in a range of approximately 0.01 mass % to 1 mass % of thetotal amount of the processing liquid.

The pH adjuster is used for adjusting pH of the processing liquid tofall within a range of 3 to 9. At the pH of less than 3,anticorrosiveness may be deteriorated. At the pH of more than 9, siliconmay be corroded. Examples of the pH adjuster are an organic acid such asacetic acid, malic acid and citric acid, a salt of the organic acid,phosphoric acid and a salt thereof.

EXAMPLES

Next, the invention will be described in detail with reference toExamples and Comparatives, but the scope of the invention is by no meanslimited by the Examples and Comparatives.

Examples 1 to 6, Comparatives 1 to 3

Aqueous processing liquids (sample liquids) of compositions shown inTable 1 were prepared and subjected to the following cut processing forevaluation. Evaluation results are also shown in Table 1.

TABLE 1 Exam- Exam- Exam- Exam- Exam- Exam- Compar- Compar- Compar- ple1 ple 2 ple 3 ple 4 ple 5 ple 6 ative 1 ative 2 ative 3 Composition ofEthyleneoxide-propyleneoxide block 1.0 3.0 6.3 8.3 — 10.5 — 12.5 —Sample Liquids copolymer (Mw13,000) *1 (mass %) Polyethylene Glycol(Mw20,000) *2 — — — — 6.0 — — — — Polyacrylic Acid (Mw798,000) *3 — — —— — — — — 1.5 Ion-Exchange Water 99.0 97.0 93.7 91.7 94.0 89.5 100.087.5 98.5 Total 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0Viscosity of (25° C. mPa · s) 1.5 2.9 5.1 8.1 8.2 12.0 1.0 20.5 5.1Sample Liquids Evaluation Accuracy of Silicon Wafer (TTV) (μm) 43 19 2933 32 40 broken 62 48 Results Accuracy of Silicon Wafer (SORI) (μm) 4236 38 48 47 50 broken 60 55 *1 ethyleneoxide-propyleneoxide blockcopolymer having a mass average molecular weight of 13,000 *2polyethylene glycol having a mass average molecular weight of 20,000 *3polyacrylic acid having a mass average molecular weight of 798,000

Processing Method

A silicon ingot was cut by a fixed abrasive grain wire saw to obtain asilicon wafer while pouring the sample liquid over the wire saw, so thata silicon wafer was obtained. Specific conditions were as follows.

Cutter: WSD-K2 (manufactured by Takatori Corporation)

Wire: electrodeposited diamond wire (diameter: 0.08 mm, grain size: 8-16μm)

Workpiece (ingot): polycrystal silicon (125 mm square)

Tension: 10 N

Wire Running Speed: 700 m/min

Supply of New Wire: 0.2 m/min

Duration of Time at Constant Speed of Wire: 10 s

Duration of Time at Acceleration and Deceleration of Wire: 3 s

Cutting distance: 133 mm

Evaluation Method

A flatness (total thickness variation (TTV)) and a warpage (SORI) of thewafer obtained by the above-described cut processing were measured.

The flatness (TTV) was obtained by measuring a thickness of the waferusing a dial gauge and was represented by a difference between a maximumthickness and a minimum thickness. In Examples, DIGIMATIC INDICATORID-C112CX manufactured by Mitutoyo Corporation was used as the dialgauge.

The warpage (SORI) is a parameter measured by a method defined by thetechnical standard QIAJ-B-007 established in Feb. 10, 2000 by QuartzCrystal Industry Association of Japan and represents undulation of thewafer in an unclamped condition by a maximum value of deviation from aflat surface (reference flat surface) in contact with a rear surface ofthe wafer. In Examples, the warpage (SORI) was measured using Nanometro44F manufactured by KURODA Precision Industries Ltd.

A viscosity of the processing liquid was measured using B-type rotaryviscometer TVB-10 (manufactured by TOKI SANGYO CO., LTD).

Evaluation Results

Since the sample liquids of Examples 1 to 6 contained a predeterminedwater-soluble polymer of the invention and the viscosity of each of thesample liquids was within a predetermined range, accuracy (TTV and SORT)of the cut silicon wafers was excellent. Moreover, the deflection of thewire during the cut processing was significantly decreased although thediameter of the wire was extremely small as much as 0.08 mm.

In contrast, in Comparative 1 in which the sample liquid was 100 mass %of ion-exchange water containing no water-soluble polymer, the wire wasbroken during the cut processing. In Comparative 2, since the viscosityof the sample liquid was excessively high, the accuracy (TTV and SORI)of the silicon wafer was extremely poor. In Comparative 3, since themass average molecular weight of the contained water-soluble polymer wasexcessively high although the viscosity of the sample liquid was withina predetermined range, accuracy (TTV and SORI) of the silicon wafer wasalso poor.

INDUSTRIAL APPLICABILITY

An aqueous processing liquid of the invention is suitable for cutting afragile material such as a silicon ingot using a fixed abrasive grainmulti-wire saw.

1. An aqueous processing liquid used for cutting a fragile materialusing a wire saw, comprising: a water-soluble polymer, wherein thewater-soluble polymer has a mass average molecular weight in a range of3×10³ to 6×10⁵, and the aqueous processing liquid exhibits a viscosityat 25 degrees C. in a range of 1.5 mPa·s to 15 Pa·s.
 2. The aqueousprocessing liquid according to claim 1, wherein the wire saw is a fixedabrasive grain wire saw.
 3. The aqueous processing liquid according toclaim 2, wherein the fixed abrasive grain wire saw has a wire diameterof 0.2 mm or less.
 4. The aqueous processing liquid according to claim1, wherein a content of the water-soluble polymer is in a range of 0.5mass % to 40 mass % of a total amount of the processing liquid.
 5. Theaqueous processing liquid according to claim 1, further comprising: atleast one of a rust inhibitor, friction modifier, antifoaming agent,metal deactivator, bactericide (preservative) and pH adjuster.
 6. Theaqueous processing liquid according to claim 1, wherein the fragilematerial is a silicon ingot.